(i) Field of the Invention
This invention relates to the treatment of pulping liquors and bleaching effluents to recover values.
(ii) Description of Prior Art
The recovery system often represents the ultimate production bottle-neck of the kraft pulping process due to its high capital cost. The purpose of the recovery system is to regenerate NaOH and Na.sub.2 S, the active pulping chemicals, from the sodium and sulphur in the spent cooking liquor and to utilize the energy value of the spent liquor organic material to produce steam.
The key equipment in the conventional recovery process is the recovery furnace where the sodium and sulphur content of the black liquor are reduced to Na.sub.2 S, and the organic solids are combusted to produce heat for steam generation. In the furnace, sodium also combines with the organic material to produce Na.sub.2 CO.sub.3, which is then causticized with lime. Lime is produced by calcining calcium carbonate, which is recovered during the causticizing operation, in a kiln. The fossil fuel requirement of the lime kiln is one of the principle obstacles preventing kraft pulp mill from becoming independent of purchased fuels.
The reduction capacity of a typical furnace is usually far greater than the combustion and heat recovery capacities, and studies have shown that furnaces can operate at inorganic/organic ratios considerably higher than those found in black liquor. Removal of a portion of the organic material from the spent liquor could therefore be an attractive means of providing incremental recovery capacity for a kraft pulp mill.
The only proven method for unloading a recovery furnace, without chemical losses, is the fluidized bed incineration of a portion of the black liquor. In this system a portion of the black liquor, at approximately 30% solids, is diverted from the evaporators and fired into a fluidized bed incinerator, where the organic material burns and pellets of a mixture of Na.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3 are recovered. The 0.3-2 mm pellets are then fed into the reducing zone of the recovery furnace. This system, however, has two distinct disadvantages. First, the fluidized bed has a poor steam generating capacity and low overall energy efficiency due to the low solids content of the feed, typically 30 to 35%, by weight, which is required to avoid excessive bed temperature and partial fusion. Secondly, the sodium sulphate reduction efficiency in the recovery furnace is adversely affected because of the non-homogenity of the feed and the very high sulphate load resulting from this mode of operation.
Another method which has been proposed for removing organic material from black liquor is acid precipitation. This process, however, has the disadvantages of having high sodium losses, significant chemical consumption and yield of a precipitate with a high moisture content.
In the patent and technical literature, methods based on electrolysis have been described to obtain essentially pure sodium hydroxide and organic lignin-based material from kraft black liquor.
Kennedy, U.S. Pat. No. 2,905,604 describes a process designed to increase the alkalinity of black liquor for recycle to the digesters and teaches an apparatus consisting of a revolving drum, which forms the anode, in a basin, which forms the cathode, of black liquor. Black liquor is continuously fed into the basin where lignin deposits on the anode drum, and NaOH forms near the basin cathode, thus yielding a liquor of increased alkalinity. Lignin is continuously scraped from the drum, washed and dried.
U.S. Pat. No. 4,584,076 Edel et al describes tests carried out with sulphur-free soda spent liquors treated in an electrolysis cell, consisting of an anode and a cathode compartment separated by a cation selective membrane. In a two-stage experimental set up the anolyte pH was reduced from 13.6 to 9.5 and subsequently to a pH of 5. Lignin was recovered from the foam layer of the anolyte chamber by drying and sodium hydroxide was recovered into a 0.1 N NaOH solution of the catholyte chamber.
The electrolytic methods described in the prior art do not provide a solution for the recovery of sulphur in usable form, i.e., as Na.sub.2 S, and hence cannot provide a viable alternative to the kraft recovery process. There remains, however, a need in the art for an energy efficient and cost-effective incremental kraft recovery process in order to alleviate the problem of overloaded recovery systems.
According to the best knowledge of the Inventors, there is no directly relevant prior art to the specific application of electrolytic recovery of NaOH and other values (lignin, other organic compounds, H.sub.2 SO.sub.4, H.sub.2, O.sub.2 or O.sub.2 /Cl.sub.2 mixture) from the process streams or combination thereof covered in this disclosure.
Similarly the Inventors have no knowledge of any description or proposed use of the techniques of this invention for enabling pulp mills to implement an oxygen delignification step between pulping and bleaching without having to overload or replace their existing pulping chemicals recovery system composed of evaporators, recovery furnace, lime kiln and causticizing plants.